To detect abnormality, such as deterioration, of a rechargeable battery, a known method determines whether or not an abnormality exists based on voltage changes during charging or discharging. This method allows the state of the battery to be evaluated without damaging the battery. Thus, the rechargeable battery may be reused when determined as being normal.
As a method for detecting an abnormal state based on the voltage of a rechargeable battery, Patent Document 1 discloses a method for comparing the battery voltage with a predetermined value when the depth of discharge of an assembled battery is 0% to 60%. It is an object of the method to detect an abnormality such as liquid depletion in the battery. The method measures the voltage for each block of the assembled battery when the depth of discharge of the assembled battery is 0% to 60%. When the voltage is less than or equal to a predetermined value, it is determined that an abnormality has occurred. The voltage used for the determination is measured when the depth of discharge is 0% to 60%, that is, when the state of charge is 40% to 100%. This is because the voltage becomes low in the final stage of discharging even in a normal battery. Thus, non-defective products are difficult to distinguish from defective products when referring to only the voltage.